Current drive circuit and display device using same, pixel circuit, and drive method

ABSTRACT

A display device including a current drive circuit capable of stably and correctly supplying an intended current to a light emitting element of each pixel without being affected by variations in characteristics of an active element inside the pixel and as a result capable of displaying a high quality image, wherein each pixel comprises a receiving use transistor TFT 3  for fetching a signal current Iw from a data line DATA when a scanning line SCAN-A is selected, a conversion use transistor TFT 1  for once converting a current level of a fetched signal current Iw to a voltage level and holding the same, and a drive use transistor TFT 2  for passing a drive current having a current level in accordance with the held voltage level through a light emitting element OLED. The conversion use thin film transistor TFT 1  generates a converted voltage level at its own gate by passing the signal current Iw fetched by the TFT 3  through its own channel. A capacitor C holds the voltage level created at the gate of the TFT 1 . The TFT 2  passes the drive current having a current level in accordance with the held voltage level through the light emitting element OLED.

TECHNICAL FIELD

The present invention relates to a current drive circuit for driving anorganic electroluminescence (EL) element or other light emitting elementcontrolled in brightness by a current, a display device providing alight emitting element driven by this current drive circuit for everypixel, a pixel circuit, and a method for driving a light emittingelement. In more detail, the present invention relates to a currentdrive circuit for controlling an amount of the current supplied to alight emitting element by an insulating gate type field effecttransistor or other active element provided in each pixel and aso-called active matrix type image display device using the same.

BACKGROUND ART

In general, in an active matrix type image display device, an image isdisplayed by arranging a large number of pixels in a matrix andcontrolling a light intensity for every pixel in accordance with givenbrightness information. When using a liquid crystal as anelectro-optical substance, the transmittance of each pixel varies inaccordance with a voltage written into the pixel. In an active matrixtype image display device using an organic electroluminescence (EL)material as the electro-optical substance as well, the basic operationis similar to that of the case where a liquid crystal is used. However,unlike a liquid crystal display, an organic EL display is a so-calledself-luminescent type having a light emitting element for every pixel,so has the advantages of a better visual recognition of the image incomparison with a liquid crystal display, no need for back light, and afast response speed. The brightnesses of individual light emittingelements are controlled by the amount of current. Namely, this displayis largely different from a liquid crystal display in the point that thelight emitting elements are current driven types or current controlledtypes.

In the same way as a liquid crystal display, in an organic EL display aswell, there are a simple matrix and an active matrix drive methods. Theformer is simple in structure, but makes it difficult to realize a largesized, high definition display, so the active matrix method is beingvigorously developed. The active matrix method controls the currentflowing through the light emitting element provided in each pixel by anactive element (generally a thin film transistor, one type of theinsulating gate type field effect transistor, hereinafter sometimesreferred to as a “TFT”) provided inside the pixel. An organic EL displayof this active matrix method is disclosed in for example JapaneseUnexamined Patent Publication (Kokai) No. 8-234683. One pixel's worth ofan equivalent circuit is shown in FIG. 1. The pixel is comprised of alight emitting element OLED, a first thin film transistor TFT1, a secondthin film transistor TFT2, and a holding capacitor C. The light emittingelement is an organic electroluminescence (EL) element. An organic ELelement has a rectification property in many cases, so is sometimesreferred to as an OLED (organic light emitting diode). In the figure,the symbol of a diode is used to indicate the light emitting elementOLED. However, the light emitting element is not always limited to anOLED and may be any element controlled in brightness by the amount ofthe current flowing through it. Also, a rectification property is notalways required in the light emitting element. In the illustratedexample, a source of the TFT2 is set at a reference potential (groundpotential), an anode of the light emitting element OLED is connected toVdd (power supply potential), and a cathode is connected to a drain ofthe TFT2. On the other hand, a gate of the TFT1 is connected to ascanning line SCAN, the source is connected to a data line DATA, and thedrain is connected to the holding capacitor C and the gate of the TFT2.

In order to operate the pixel, first, when the scanning line SCAN isbrought to a selected state and a data potential Vw representing thebrightness information is applied to the data line DATA, the TFT1becomes conductive, the holding capacitor C is charged or discharged,and the gate potential of the TFT2 coincides with the data potential Vw.When the scanning line SCAN is brought to an unselected state, the TFT1becomes OFF and the TFT2 is electrically separated from the data lineDATA, but the gate potential of the TFT2 is stably held by the holdingcapacitor C. The current flowing through the light emitting element OLEDvia the TFT2 becomes a value in accordance with a gate/source voltageVgs, and the light emitting element OLED continuously emits the lightwith a brightness in accordance with the amount of the current suppliedthrough the TFT2.

When the current flowing between the drain and source of the TFT2 isIds, this is the drive current flowing through the OLED. Assuming thatthe TFT2 operates in the saturated region, Ids is represented by thefollowing equation. $\begin{matrix}\begin{matrix}{{Ids} = {{\mu \cdot {Cox} \cdot {{W/L}/2}}\left( {{Vgs} - {Vth}} \right)^{2}}} \\{= {{\mu \cdot {Cox} \cdot {{W/L}/2}}\left( {{Vw} - {Vth}} \right)^{2}2}}\end{matrix} & (1)\end{matrix}$

Here, Cox is the gate capacity per unit area and is given by thefollowing equation:Cox=ε0·εr/d  (2)

In equation (1) and equation (2), Vth indicates a threshold value of theTFT2, μ indicates a mobility of a carrier, W indicates a channel width,L indicates a channel length, ε0 indicates a permittivity of vacuum, εrindicates a dielectric constant of the gate insulating film, and d is athickness of the gate insulating film.

According to equation (1), Ids can be controlled by the potential Vwwritten into the pixel. As a result, the brightness of the lightemitting element OLED can be controlled. Here, the reason for theoperation of the TFT2 in the saturated region is as follows. Namely,this is because, in the saturated region, Ids is controlled by only theVgs and does not depend upon the drain/source voltage Vds. Therefore,even if Vds fluctuates due to variations in the characteristics of theOLED, a predetermined amount of the drive current Ids can be passedthrough the OLED.

As mentioned above, in the circuit configuration of the pixel shown inFIG. 1, when written by Vw once, the OLED continues emitting light witha constant brightness during one scanning cycle (one frame) until nextrewritten. If large number of such pixels are arranged in a matrix as inFIG. 2, an active matrix type display device can be configured. As shownin FIG. 2, in a conventional display device, scanning lines SCAN-1through SCAN-N for selecting pixels 25 in a predetermined scanning cycle(for example a frame cycle according to an NTSC standard) and data linesDATA giving brightness information (data potential Vw) for driving thepixels 25 are arranged in a matrix. The scanning lines SCAN-1 throughSCAN-N are connected to a scanning line drive circuit 21, while the datalines DATA are connected to a data line drive circuit 22. By repeatingthe writing of Vw from the data lines DATA by the data line drivecircuit 22 while successively selecting the scanning lines SCAN-1through SCAN-N by the scanning line drive circuit 21, an intended imagecan be displayed. In a simple matrix type display device, the lightemitting element contained in each pixel emits light only at an instantof selection. In contrast, in the active matrix type display deviceshown in FIG. 2, the light emitting element of each pixel 25 continuesto emit light even after finishing being written. Therefore, inparticular in a large sized, high definition display, there is theadvantage that the level of the drive current of the light emittingelements can be lowered in comparison with the simple matrix type.

FIG. 3 schematically shows a sectional structure of the pixel 25 shownin FIG. 2. Note, only OLED and TFT2 are represented for facilitating theillustration. The OLED is configured by successively superposing atransparent electrode 10, an organic EL layer 11, and a metal electrode12. The transparent electrode 10 is separated for every pixel, acts asthe anode of the OLED, and is made of a transparent conductive film forexample ITO. The metal electrode 12 is commonly connected among pixelsand acts as the cathode of the OLED. Namely, the metal electrode 12 iscommonly connected to a predetermined power supply potential Vdd. Theorganic EL layer 11 is a composite film obtained by superposing forexample a positive hole transport layer and an electron transport layer.For example, Diamyne is vapor deposited on the transparent electrode 10acting as the anode (positive hole injection electrode) as the positivehole transport layer, Alq3 is vapor deposited thereon as the electrontransport layer. Further, a metal electrode 12 acting as the cathode(electron injection electrode) is grown thereon. Note that, Alq3represents 8-hydroxy quinoline aluminum. The OLED having such a laminatestructure is only one example. When a voltage in a forward direction(about 10V) is applied between the anode and the cathode of the OLEDhaving such a configuration, injection of carriers such as electrons andpositive holes occurs and luminescence is observed. The operation of theOLED can be considered to be the emission of light by excisions formedby the positive holes injected from the positive hole transport layerand the electrons injected from the electron transport-layer.

On the other hand, the TFT2 comprises a gate electrode 2 formed on asubstrate 1 made of glass or the like, a gate insulating film 3superimposed on the top surface thereof, and a semiconductor thin film 4superimposed above the gate electrode 2 via this gate insulating film 3.This semiconductor thin film 4 is made of for example a polycrystallinesilicon thin film. The TFT2 is provided with a source S, a channel Ch,and a drain D acting as a passage of the current supplied to the OLED.The channel Ch is located immediately directly above the gate electrode2. The TFT2 of this bottom gate structure is coated by an inter-layerinsulating film 5. A source electrode 6 and a drain electrode 7 areformed above that. Above them, the OLED mentioned above is grown viaanother inter-layer insulating film 9. Note that, in the example of FIG.3, the anode of the OLED is connected to the drain of the TFT2, so aP-channel thin film transistor is used as the TFT2.

In an active matrix type organic EL display, generally a TFT (thin filmtransistor) formed on a glass substrate is utilized as the activeelement. This is for the following reason. Namely, an organic EL displayis a direct viewing type. Due to this, it becomes relatively large insize. Due to restrictions of cost and manufacturing facilities, a usageof a single crystalline silicon substrate for the formation of theactive elements is not practical. Further, in order to extract the lightfrom the light emitting elements, usually a transparent conductive filmof ITO (indium tin oxide) is used as the anode of the organic EL layer,but ITO is frequently generally grown under a high temperature which anorganic EL layer cannot endure. In this case, it is necessary to formthe ITO before the formation of the organic EL layer. Accordingly, themanufacture process roughly becomes as follows:

Referring to FIG. 3 again, first the gate electrode 2, gate insulatingfilm 3, and semiconductor thin film 4 comprised of amorphous silicon aresuccessively stacked and patterned on the glass substrate 1 to form theTFT2. In certain cases, the amorphous silicon is sometimes formed intopolysilicon (polycrystalline silicon) by heat treatment such as laserannealing. In this case, generally a TFT2 having a larger degree ofcarrier mobility in comparison with amorphous silicon and a largercurrent driving capability can be formed. Next, an ITO transparentelectrode 10 acting as the anode of the light emitting element OLED isformed. Subsequently, an organic EL layer 11 is stacked to form thelight emitting element OLED. Finally, the metal electrode 12 acting asthe cathode of the light emitting element is formed by a metal material(for example aluminum).

In this case, the extraction of the light is started from a back side(bottom surface side) of the substrate 1, so a transparent material(usually a glass) must be used for the substrate 1. In view of this, inan active matrix type organic EL display, a relatively large sized glasssubstrate 1 is used. As the active element, ordinarily use is made of aTFT as it can be relatively easily formed thereon. Recently, attemptshave also been made to extract the light from a front side (top surfaceside) of the substrate 1. The sectional structure in this case is shownin FIG. 4. The difference of this from FIG. 3 resides in that the lightemitting element OLED is comprised by successively superposing a metalelectrode 12 a, an organic EL layer 11, and a transparent electrode 10 aand an N-channel transistor is used as the TFT2.

In this case, the substrate 1 does not have to be transparent likeglass, but as the transistor formed on a large sized substrate, use isgenerally still made of a TFT. However, the amorphous silicon andpolysilicon used for the formation of the TFT have a worse crystallinityin comparison with single crystalline silicon and have a poorcontrollability of the conduction mechanism, therefore it has been knownthat there is a large variation in characteristics in formed TFTs.Particularly, when a polysilicon TFT is formed on a relatively largesized glass substrate, usually the laser annealing method is used asmentioned above in order to avoid the problem of thermal deformation ofthe glass substrate, but it is difficult to uniformly irradiate laserenergy to a large glass substrate. Occurrence of variations in the stateof the crystallization of the polysilicon according to the location inthe substrate cannot be avoided.

As a result, it is not rare for the Vth (threshold value) to varyaccording to pixel by several hundreds of mV, in certain cases, 1V ormore, even in the TFTs formed on an identical substrate. In this case,even if a same signal potential Vw is written with respect to forexample different pixels, the Vth will vary according to the pixels. Asa result, according to equation (1) described above, the current Idsflowing through the OLEDs will largely vary for every pixel andconsequently become completely off from the intended value, so a highquality of image cannot be expected as the display. A similar thing canbe said for not only the Vth, but also the variation of parameters ofequation (1) such as the carrier mobility μ. Further, a certain degreeof fluctuation in the above parameters is unavoidable not only due tothe variation among pixels as mentioned above, but also variations forevery manufacturing lot or every product. In such a case, it isnecessary to determine how the data line potential Vw should be set withrespect to the intended current Ids to be passed through the OLEDs forevery product in accordance with the final state of the parameters ofequation (1). Not only is this impractical in the mass productionprocess of displays, but it is also extremely difficult to devisecountermeasures for fluctuations in characteristics of the TFTs due tothe ambient temperature and changes of the TFT characteristics occurringdue to usage over a long period of time.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a current drive circuitcapable of stably and accurately supplying an intended current to alight emitting element etc. of a pixel without being affected byvariations in characteristics of an active element inside the pixel, adisplay device using the same and as a result capable of displaying ahigh quality image, a pixel circuit, and a method for driving a lightemitting element.

In order to achieve the object, the following means were devised.Namely, a display device according to the present invention provides ascanning line drive circuit for successively selecting scanning lines, adata line drive circuit including a current source for generating asignal current having a current level in accordance with brightnessinformation and successively supplying the same to data lines, and aplurality of pixels arranged at intersecting portions of the scanninglines and the data lines and including current driven type lightemitting elements emitting light by receiving the supply of the drivecurrent. The characterizing feature is that each pixel comprises areceiving part for fetching the signal current from the data line whenthe scanning line is selected, a converting part for converting acurrent level of the fetched signal current to a voltage level andholding the same, and a drive part for passing a drive current having acurrent level in accordance with the held voltage level through thelight emitting element. Specifically, the converting part includes aconversion use insulating gate type field effect transistor providedwith a gate, a source, a drain, and a channel and a capacitor connectedto the gate. The conversion use insulating gate type field effecttransistor generates a converted voltage level at the gate by passingthe signal current fetched by the receiving part through the channel.The capacitor holds the voltage level created at the gate. Further, theconverting part includes a switch use insulating gate type field effecttransistor inserted between the drain and the gate of the conversion useinsulating gate type field effect transistor. The switch use insulatinggate type field effect transistor becomes conductive when converting thecurrent level of the signal current to the voltage level andelectrically connects the drain and the gate of the conversion useinsulating gate type field effect transistor to create the voltage levelwith the source as the reference at the gate, while the switch useinsulating gate type field effect transistor is shut off when thecapacitor holds the voltage level and separates the gate of theconversion use insulating gate type field effect transistor and thecapacitor connected to this from the drain.

In one embodiment, the drive part includes a drive use insulating gatetype field effect transistor provided with a gate, a drain, a source,and a channel. This drive use insulating gate type field effecttransistor receives the voltage level held at the capacitor at its gateand passes a drive current having a current level in accordance withthat through the light emitting element via the channel. A currentmirror circuit is configured by direct connection of the gate of theconversion use insulating gate type field effect transistor and the gateof the drive use insulating gate type field effect transistor, whereby aproportional relationship is exhibited between the current level of thesignal current and the current level of the drive current. The drive useinsulating gate type field effect transistor is formed in the vicinityof the corresponding conversion use insulating gate type field effecttransistor inside the pixel and has an equivalent threshold voltage tothat of the conversion use insulating gate type field effect transistor.The drive use insulating gate type field effect transistor operates inthe saturated region and passes a drive current in accordance with adifference between the level of the voltage applied to the gate thereofand the threshold voltage through the light emitting element.

In another embodiment, the drive part shares the conversion useinsulating gate type field effect transistor together with theconverting part in a time division manner. The drive part separates theconversion use insulating gate type field effect transistor from thereceiving part and uses the same for driving after the conversion of thesignal current is completed and passes the drive current to the lightemitting element through the channel in a state where the held voltagelevel is applied to the gate of the conversion use insulating gate typefield effect transistor. The drive part has a controlling means forcutting off an unnecessary current flowing to the light emitting elementvia the conversion use insulating gate type field effect transistor attimes other than the time of drive. The controlling means cuts off theunnecessary current by controlling a voltage between terminals of a twoterminal type light emitting element having a rectification function.Alternatively, the controlling means comprises a control use insulatinggate type field effect transistor inserted between the conversion useinsulating gate type field effect transistor and the light emittingelement, and the control use insulating gate type field effecttransistor becomes nonconductive in state and separates the conversionuse insulating gate type field effect transistor and the light emittingelement when the light emitting element is not driven and switches tothe conductive state when the light emitting element is driven. Inaddition, the controlling means controls a ratio between a time forcutting off the drive current when the light emitting element is not tobe driven and placing the light emitting element in the non-lightemitting state and a time of passing the drive current when the lightemitting element is to be driven and placing the light emitting elementin the light emitting and thereby to enable the control of thebrightness of the pixel. According to a certain case, the drive part hasa potential fixing means for fixing the potential of the drain withreference to the source of the conversion use insulating gate type fieldeffect transistor in order to stabilize the current level of the drivecurrent flowing to the light emitting element through the conversion useinsulating gate type field effect transistor.

In a further developed form of the present invention, the receivingpart, the converting part, and the drive part configure a currentcircuit combining a plurality of insulating gate type field effecttransistors, and one or two or more insulating gate type field effecttransistors have a double gate structure for suppressing current leakagein the current circuit. Further, the drive part includes the insulatinggate type field effect transistor provided with the gate, drain, and thesource and passes the drive current passing between the drain and thesource to the light emitting element in accordance with the level of thevoltage applied to the gate, the light emitting element is a twoterminal type having an anode and a cathode, and the cathode isconnected to the drain. Alternatively, the drive part includes aninsulating gate type field effect transistor provided with a gate, adrain, and a source and passes a drive current passing between the drainand the source to the light emitting element in accordance with thelevel of the voltage applied to the gate, the light emitting element isa two terminal type having an anode and a cathode, and the anode isconnected to the source. Further, it includes an adjusting means fordownwardly adjusting the voltage level held by the converting part andsupplying the same to the drive part to tighten the black level of thebrightness of each pixel. In this case, the drive part includes aninsulating gate type field effect transistor having a gate, a drain, anda source, and the adjusting means downwardly adjusts the level of thevoltage applied to the gate by raising the bottom of the voltage betweenthe gate and the source of the insulating gate type field effecttransistor. Alternatively, the drive part includes an insulating gatetype field effect transistor having a gate, a drain, and a source, theconverting part is provided with a capacitor connected to the gate ofthe thin film transistor and holding the voltage level, and theadjusting means comprises an additional capacitor connected to thatcapacitor and downwardly adjusts the level of the voltage to be appliedto the gate of the insulating gate type field effect transistor held atthat capacitor. Alternatively, the drive part includes an insulatinggate type field effect transistor having a gate, a drain, and a source,the converting part is provided with a capacitor connected to the gateof the insulating gate type field effect transistor on its one end andholding the voltage level, and the adjusting means adjusts the potentialof the other end of the capacitor when holding the voltage levelconverted by the converting part at that capacitor to downwardly adjustthe level of the voltage to be applied to the gate of the insulatinggate type field effect transistor. Note that, as the light emittingelement, use is made of for example an organic electroluminescenceelement.

The pixel circuit of the present invention has the followingcharacteristic features. First, the brightness information is written toa pixel by passing a signal current having a magnitude in accordancewith the brightness through the data line. That current flows betweenthe source and the drain of the conversion use insulating gate typefield effect transistor inside the pixel and as a result creates avoltage between the gate and source in accordance with the currentlevel. Second, the voltage between the gate and source created asdescribed above or the gate potential is held by the function of thecapacitor formed inside the pixel or existing parasitically and is heldat about that level for a predetermined period even after the end of thewriting. Third, the current flowing through the OLED is controlled bythe conversion use insulating gate type field effect transistor per seconnected to it in series or the drive use insulating gate type fieldeffect transistor provided inside the pixel separately from that andhaving a gate commonly connected together with the conversion useinsulating gate type field effect transistor. The voltage between thegate and source at the OLED drive is generally equal to the voltagebetween the gate and source of the conversion use insulating gate typefield effect transistor created according to the first characterizingfeature. Fourth, at the time of writing, the data line and the internalportion of the pixel are made conductive by a fetch use insulating gatetype field effect transistor controlled by the first scanning line, andthe gate and the drain of the conversion use insulating gate type fieldeffect transistor are short-circuited by the switch use insulating gatetype field effect transistor controlled by the second scanning line.Summarizing the above, while in the conventional example, the brightnessinformation was given in the form of a voltage value, in contrast, theremarkable characterizing feature of the display device of the presentinvention is that the brightness information is given in the form of acurrent value, that is, of a current written type.

As already mentioned, an object of the present invention is toaccurately pass the intended current through the OLEDs without beingaffected by variations in the characteristics of the TFTs. The reasonwhy the present object can be achieved by the first through fourthcharacterizing features will be explained below. Note that hereinafterthe conversion use insulating gate type field effect transistor will bedescribed as the TFT1, the drive use insulating gate type field effecttransistor will be described as the TFT2, the fetch use insulating gatetype field effect transistor will be described as the TFT3, and theswitch use insulating gate type field effect transistor will bedescribed as the TFT4. Note that the present invention is not limited toTFTs (thin film transistors). Insulating gate type field effecttransistors can be widely employed as the active elements, for example,single crystalline silicon transistors formed on a single crystallinesilicon substrate or SOI substrate. The signal current passing throughthe TFT1 at the time of writing of the brightness information is definedas Iw, and the voltage between the gate and source created in the TFT1as a result of this is defined as Vgs. At the time of writing, due tothe TFT4, the gate and the drain of the TFT1 are short-circuited, so theTFT1 operates in the saturated region. Accordingly, Iw is given by thefollowing equation.Iw=μ1·Cox1·W1/L1/2(Vgs−Vth1)²  (3)

Here, the meanings of the parameters are similar to the case of equation(1). Next, when defining the current flowing through an OLED as Idrv,Idrv is controlled in its current level by the TFT2 connected to theOLED in series. In the present invention, the voltage between the gateand source thereof coincides with Vgs in equation (3). Therefore, whenassuming that the TFT2 operates in the saturated region, the followingequation stands:Idrv=μ2·Cox2·W2/L2/2(Vgs−Vth2)²  (4)

The meanings of the parameters are similar to the case of equation (1).Note that, the condition for the operation of the insulating gate typefield effect transistor in the saturated region is generally given bythe following equation while defining Vds as the voltage between thedrain and source.|Vds|>|Vgs−Vth|  (5)

Here, TFT1 and TFT2 are formed close inside a small pixel, so it can beconsidered that de facto μ1=μ2, Cox1=Cox2, and Vth1=Vth2. Then, at thistime, the following equation is easily derived from equation (3) andequation (4):Idrv/Iw=(W2/L2)/(W1/L1)  (6)

The point to be noted here resides in the fact that, in equation (3) andequation (4), the values of μ, Cox, and Vth per se vary for every pixel,every product, or every manufacturing lot, but equation (6) does notinclude these parameters, so the value of Idrv/Iw is not affected bysuch variation of them. For example, when designing W1=W2 and L1=L2,Idrv/Iw=1 stands, that is, Iw and Idrv become an identical value.Namely, the drive current Idrv flowing through the OLED becomesaccurately identical to the signal current Iw without being affected byvariations in the characteristics of the TFT. Therefore, as a result,the light emitting brightness of the OLED can be accurately controlled.The above description is just one example. As will be explained below bygiving embodiments, the ratio of Iw and Idrv can be freely determinedaccording to how W1, W2, L1, and L2 are set. Alternatively, it is alsopossible to use the same TFT for the TFT1 and TFT2.

In this way, according to the present invention, the correct current canbe passed through the OLED without being affected by variations in thecharacteristics of the TFT. Further, according to equation (6), there isthe large advantage of the simple proportional relationship between Iwand Idrv. Namely, in the conventional example of FIG. 1, as shown inequation (1), Vw and Idrv are nonlinear and are affected by variationsin the characteristics of the TFT, so the control of the voltage at thedrive side becomes complex. Further, it is seen that the carriermobility μ among the characteristics of the TFT shown in equation (1)fluctuates according to the temperature. In this case, in theconventional example, according to equation (1), Idrv, and accordinglythe light emitting brightness of the OLED, changes, but according to thepresent invention, such a worry does not exist. The value of Idrv givenby equation (6) can be stably supplied to the OLED.

In equation (4), it was assumed that the TFT2 operated in the saturatedregion, but the present invention is effective in also a case where theTFT2 operates in a linear region. Namely, where the TFT2 operates in thelinear region, Idrw is given by the following equation:Idrv=μ2·Cox2·W2/L2*{(Vgs−Vth2)Vds2−Vds2²/2}  (7)

Vds2 is the voltage between the drain and source of TFT2. Here, whenassuming that TFT1 and TFT2 are arranged close and as a resultVth1=Vth2=Vth stands, Vgs and Vth can be deleted from equation (3) andequation (7) and the following equation is obtained:Idrv=μ2·Cox2·W2/L2*{(2Iw·L1/μ1·Cox1·W1)^(1/2) Vds2−Vds2²/2}  (8)

In this case, the relationship between Iw and Idrv does not become asimple proportional relationship as in equation (6), but Vth is notcontained in equation (8). Therefore, it is seen that the relationshipof Iw and Idrv is not affected by the variation of Vth (variation in ascreen or variation for every manufacturing lot) Namely, by writing thepredetermined Iw without being affected by variation of the Vth, theintended Idrv can be obtained. Note, where μ and Cox vary in the screen,due to these values, even if a specific Iw is given to the data line,the value of Idrv determined from equation (8) will vary. Thereforedesirably the TFT2 operates in the saturated region as mentioned before.

Further, more preferably the TFT3 and the TFT4 are controlled bydifferent scanning lines, and the TFT4 is brought to the off statepreceding the TFT3 at the end of the write operation. In the pixelcircuit according to the present invention, the TFT3 and the TFT4 do nothave to be the same conductivity type. The pixel circuit may beconfigured so that the TFT3 and the TFT4 are an identical or differentconductivity types, the gates of them controlled by different scanninglines, and the TFT4 brought to the off state preceding to the TFT3 atthe end of the write operation.

Further, when the TFT3 and the TFT4 are controlled by different scanninglines, after the end of the write operation, the TFT4 may be brought tothe on state by the operation of the scanning line, and the pixelsextinguished in units of the scanning lines. This is because, the gateand the drain of the TFT1 and the gate of the TFT2 are connected, so thegate voltage of the TFT2 becomes the threshold value of the TFT1 (thisis almost equal to the threshold value of the TFT2), and both of theTFT1 and TFT2 become the off state.

In this way, by changing the timing of the extinguishing signal, it ispossible to conveniently and freely change the brightness of the displaydevice. If the second scanning line is divided into colors of R, G, andB and separately controlled, adjustment of the color balance is alsoeasy.

Further, where it is desired to obtain the same time average brightness,the drive current of a light emitting element OLED can be made larger byreducing the ratio of the light emitting period (duty).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of an example of a conventional pixelcircuit.

FIG. 2 is a block diagram of an example of the configuration of aconventional display device.

FIG. 3 is a sectional view of an example of the configuration of aconventional display device.

FIG. 4 is a sectional view of another example of the configuration of aconventional display device.

FIG. 5 is a circuit diagram of an embodiment of a pixel circuitaccording to the present invention.

FIG. 6 is a waveform diagram of an example of waveforms of signals inthe embodiment of FIG. 5.

FIG. 7 is a block diagram of an example of the configuration of adisplay device using a pixel circuit according to the embodiment of FIG.5.

FIG. 8 is a circuit diagram of a modification of the embodiment of FIG.5.

FIG. 9 is a circuit diagram of another embodiment of a pixel circuitaccording to the present invention.

FIG. 10 is a waveform diagram of an example of the waveforms of signalsin the embodiment of FIG. 9.

FIG. 11 is a circuit diagram of a modification of the embodiment of FIG.9.

FIG. 12 is a circuit diagram of a modification of the embodiment of FIG.9.

FIG. 13 is a circuit diagram of a modification of the embodiment of FIG.9.

FIG. 14 is a circuit diagram of a modification of the embodiment of FIG.9.

FIG. 15 is a circuit diagram of another embodiment of the pixel circuitaccording to the present invention.

FIG. 16 is a circuit diagram of a modification of the embodiment of FIG.15.

FIG. 17 is a circuit diagram of a modification of the embodiment of FIG.15.

FIG. 18 is a circuit diagram of another embodiment of the pixel circuitaccording to the present invention.

FIG. 19 is a circuit diagram of a modification of the embodiment of FIG.18.

FIG. 20 is a view for explaining a case where the pixels areextinguished in units of scanning lines in the circuit of FIG. 19.

FIG. 21 is a circuit diagram of a modification of the embodiment of FIG.19.

FIG. 22 is a circuit diagram of a modification of the embodiment of FIG.19.

FIG. 23 is a diagram of characteristics of currents flowing throughconversion use transistors of the circuit of FIG. 22 and theconventional circuit.

FIG. 24 is a circuit diagram of a modification of the embodiment of FIG.19.

FIG. 25 is a view of data line potentials of the circuit of FIG. 23 andthe conventional circuit.

FIG. 26 is a circuit diagram of another embodiment of the pixel circuitaccording to the present invention.

FIG. 27 is a circuit diagram of another embodiment of the pixel circuitaccording to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Below, embodiments of the present invention will be explained byreferring to the attached drawings.

FIG. 5 shows an example of a pixel circuit according to the presentinvention. This circuit comprises, other than the conversion usetransistor TFT1 with the signal current flowing therethrough and thedrive use transistor TFT2 for controlling the drive current flowingthrough a light emitting element made of an organic EL element or thelike, a fetch use transistor TFT3 for connecting or disconnecting thepixel circuit and the data line DATA by the control of a first scanningline SCAN-A, a switch use transistor TFT4 for short-circuiting the gateand the drain of the TFT1 during the writing period by the control of asecond scanning line SCAN-B, a capacitor C for holding the voltagebetween the gate and source of the TFT1 even after the end of thewriting, and the light emitting element OLED. In FIG. 5, TFT3 isconfigured by a PMOS, and the other transistors are configured by NMOSs,but this is one example. The invention does not always have to be thisway. The capacitor C is connected to the gate of the TFT1 at its oneterminal, and connected to the GND (ground potential) at its otherterminal, but this is not limited to GND. Any constant potential ispossible. The anode of the OLED is connected to the positive powersupply potential Vdd.

Basically, the display device according to the present invention isprovided with a scanning line drive circuit for successively selectingscanning lines SCAN-A and SCAN-B, a data line drive circuit including acurrent source CS for generating a signal current Iw having a currentlevel in accordance with the brightness information and successivelysupplying the same to the data lines DATA, and a plurality of pixelsarranged at intersecting portions of the scanning lines SCAN-A andSCAN-B and data lines DATA and including current drive type lightemitting elements OLED emitting light by receiving the supply of thedrive current. As the characterizing feature, each pixel shown in FIG. 5comprises a receiving part for fetching the signal current Iw from thedata line DATA when the scanning line SCAN-A is selected, a convertingpart for once converting the current level of the fetched signal currentIw to the voltage level and holding the same, and a drive part forpassing the drive current having the current level in accordance withthe held voltage level through the light emitting element OLED.Specifically, the converting part includes a conversion use thin filmtransistor TFT1 provided with a gate, source, drain, and channel and thecapacitor C connected to the gate. The conversion use thin filmtransistor TFT1 generates a converted voltage level at the gate bypassing the signal current Iw fetched by the receiving part through thechannel, while the capacitor C holds the voltage level created at thegate. Further, the converting part includes the switch use thin filmtransistor TFT4 inserted between the drain and gate of the conversionuse thin film transistor TFT1. The switch use thin film transistor TFT4becomes conductive when converting the current level of the signalcurrent Iw to the voltage level, electrically connects the drain andgate of the conversion use thin film transistor TFT1, and creates thevoltage level with reference to the source at the gate of the TFT1.Further, the switch use thin film transistor TFT4 is cut off when thecapacitor C holds the voltage level and separates the gate of theconversion use thin film transistor TFT1 and the capacitor C connectedto this from the drain of the TFT1.

Further, the drive part includes a drive use thin film transistor TFT2provided with a gate, drain, source, and channel. The drive use thinfilm transistor TFT2 receives the voltage level held at the capacitor Cat its gate and passes a drive current having a current level inaccordance with that via the channel to the light emitting element OLED.A current mirror circuit is configured by direct connection of the gateof the conversion use thin film transistor TFT1 and the gate of thedrive use thin film transistor TFT2, whereby a proportional relationshipis exhibited between the current level of the signal current Iw and thecurrent level of the drive current. The drive use thin film transistorTFT2 is formed in the vicinity of the corresponding conversion use thinfilm transistor TFT1 inside the pixel and has an equivalent thresholdvoltage to that of the conversion use thin film transistor TFT1. Thedrive use thin film transistor TFT2 operates in the saturated region andpasses a drive current in accordance with the difference between thelevel of the voltage applied to the gate thereof and the thresholdvoltage to the light emitting element OLED.

The driving method of the present pixel circuit is as follows. The drivewaveforms are shown in FIG. 6. First, at the time of writing, the firstscanning line SCAN-A and the second scanning line SCAN-B are broughtinto the selected state. In the example of FIG. 6, the first scanningline SCAN-A is set at a low level, and the second scanning line SCAN-Bis set at a high level. By connecting the current source CS to the dataline DATA in a state where both scanning lines are selected, the signalcurrent Iw in accordance with the brightness information flows throughthe TFT1. The current source CS is a variable current source controlledin accordance with the brightness information. At this time, the gateand the drain of the TFT1 are short-circuited by the TFT4, and thereforeequation (5) stands, and the TFT1 operates in the saturated region.Accordingly, between the gate and the source thereof, a voltage Vgsgiven by equation (3) is created. Next, the first scanning line SCAN-Aand the second scanning line SCAN-B are brought to the unselected state.In more detail, first the second scanning line SCAN-B is set at a lowlevel and the TFT4 is brought into an off state. By this, Vgs is held bythe capacity C. Next, by setting the first scanning line SCAN-A at ahigh level and bringing it to the off state, the pixel circuit and thedata line DATA are electrically cut off, and therefore, the writing tothe other pixel can be carried out via the data line DATA thereafter.Here, the data output by the current source CS as the current level ofthe signal current must be effective at the point of time when thesecond scanning line SCAN-B becomes unselected, but after that, may beset at any level (for example the write data of the next pixel). Thegate and the source of the TFT2 are commonly connected together with theTFT1. Further, the two are formed close inside a small pixel. Therefore,if the TFT2 operates in the saturated region, the current flowingthrough the TFT2 is given by equation (4). This becomes the drivecurrent Idrv flowing through the light emitting element OLED. In orderto operate the TFT2 in the saturated region, a sufficient positivepotential may be given to the Vdd so that equation (5) still stands evenif a voltage drop at the light emitting element OLED is considered.

According to the above drive, the current Idrv flowing through the lightemitting element OLED is given by the previous equation (6):Idrv=(W2/L2)/(W1/L1)·Iwand a value correctly proportional to Iw without being affected byvariations in the characteristics of the TFT is obtained. Theproportional constant (W2/L2)/(W1/L1) can be set to a proper value byconsidering various circumstances. For example, where assuming that thevalue of the current to be passed through the light emitting elementOLED of one pixel is a relatively small value, for example 10 nA, as theactual problem, it is sometimes difficult to correctly supply such asmall current value as the signal current Iw. In such a case, if adesign is made so that (W2/L2)/(W1/L1)=1/100 stands, Iw becomes 1 μAfrom equation (6) and the current write operation becomes easy.

In the above example, it was assumed that the TFT2 operated in thesaturated region, but the present invention is effective even in thecase where the TFT2 operates in the linear region as mentioned before.Namely, where the TFT2 operates in the linear region, the current Idrvflowing through the light emitting element OLED is given by the aboveequation (8):Idrv=μ2·Cox2·W2/L2*{(2Iw·L1/μ1·Cox1·W1)^(1/2) Vds2−Vds2²/2}In the above equation, Vds2 is determined by current-voltagecharacteristics of the light emitting element OLED and the current Idrvflowing through the light emitting element OLED. When the potential ofVdd and the characteristics of the light emitting element OLED aregiven, it is a function of only Idrv. In this case, the relationshipbetween Iw and Idrv does not become the simple proportional relationshipas in equation (6), but if Iw is given, the Idrv satisfying equation (8)becomes the drive current flowing through the OLED. Vth is not containedin equation (8), therefore it is seen that the relationship between Iwand Idrv is not affected by the variation of Vth (variation for everypixel in the screen or variation for every manufacturing lot). Namely,by writing the predetermined Iw without being affected by variation inthe Vth, the intended Idrv can be obtained. In this way, when the TFT2operates in the linear region, the voltage between the drain and thesource of the TFT2 becomes small in comparison with the case where itoperates in the saturated region, therefore a low power consumption canbe realized.

FIG. 7 shows an example of the display device configured by arrangingthe pixel circuits of FIG. 5 in the matrix state. The operation thereofwill be explained below. First, a vertical start pulse (VSP) is input toa scanning line drive circuit A21 including the shift register and ascanning line drive circuit B23 similarly including the shift register.After receiving VSP, the scanning line drive circuit A21 and scanningline drive circuit B23 successively select first scanning lines SCAN-A1to SCAN-AN and second scanning lines SCAN-B1 to SCAN-BN synchronous tothe vertical clocks (VCKA, VCKB). The current source CS is provided inthe data line drive circuit 22 corresponding to each data line DATA anddrives the data line at a current level in accordance with thebrightness information. The current source CS comprises an illustratedvoltage/current conversion circuit and outputs the signal current inaccordance with the voltage representing the brightness information. Thesignal current flows through the pixel on the selected scanning line,and the current is written in units of the scanning lines. Each pixelstarts to emit light with an intensity in accordance with its currentlevel. Note, VCKA is slightly delayed relative to VCKB by a delaycircuit 24. By this, as shown in FIG. 6, SCAN-B becomes unselectedpreceding SCAN-A.

FIG. 8 is a modification of the pixel circuit of FIG. 5. This circuitgives a double gate configuration wherein two transistors TFT2 a andTFT2 b are connected in series to the TFT2 in FIG. 5 and imparts adouble gate configuration wherein two transistors TFT4 a and TFT4 b areconnected in series to the TFT4 in FIG. 5. The gates of the TFT2 a andTFT2 b and the gates of the TFT4 a and TFT4 b are commonly connected,therefore basically they perform a similar operation to that of singletransistors. As a result, also the pixel circuit of FIG. 8 performs asimilar operation to that of the pixel circuit of FIG. 5. With a singletransistor, particularly TFT, there is a case where the leakage currentat the off time becomes large according to a certain defect or the like.For this reason, when it is intended to suppress the leakage current,preferably a redundant configuration of connecting a plurality oftransistors in series is employed. This is because, when employing this,even if there is a leakage in one transistor, if the leakage of theother transistor is small, the leakage as a whole can be suppressed.When employing the configuration such as TFT2 a and TFT2 b of FIG. 8,due to the small leakage current, there arises a merit that the qualityof the black level of the display becomes good when the brightness iszero (current zero). Further, when employing the configuration such asTFT4 a and TFT4 b, there arises a merit that the brightness informationwritten in the capacitor C can be stably held. For these, similarly, itis also possible to configure three or more transistors in series. Asdescribed above, in the present modification, the receiving part,converting part, and the drive part configure the current circuitcombining a plurality of thin film transistors TFT. One or more thinfilm transistors (TFT) have the double gate structure for suppressingthe current leakage in the current circuit.

FIG. 9 shows another embodiment of the pixel circuit according to thepresent invention. The characterizing feature of this circuit resides inthat the transistor TFT1 with the signal current Iw flowing therethroughper se controls the current Idrv flowing through the light emittingelement OLED. In the pixel circuit shown in FIG. 5 mentioned before,when the characteristics of TFT1 and TFT2 (Vth, μ or the like) areslightly different from each other, equation (6) does not correctlystand, and there is a possibility such that Iw and Idrv are notcorrectly proportional, but in the pixel circuit of FIG. 9, such aproblem does not occur in principle. The pixel circuit of FIG. 9 isprovided with, other than the TFT1, a transistor TFT3 for connecting ordisconnecting the pixel circuit and the data line DATA by the control ofthe first scanning line SCAN-A, a transistor TFT4 for short-circuitingthe gate and the drain of the TFT1 during the writing period by thecontrol of the second scanning line SCAN-B, a capacitor C for holdingthe voltage between the gate and source of the TFT1 even after the endof the writing, and a light emitting element OLED made of the organic ELelement. The holding capacitor C is connected to the gate of the TFT1 atits one terminal and connected to the GND (ground potential) at itsother terminal, but this is not limited to GND. Any constant potentialis possible. The anode of the light emitting element OLED is connectedto the anode line A arranged in units of the scanning lines. The TFT3 isconfigured by a PMOS, and the other transistors are configured by NMOSs,but this is one example. The invention does not always have to be thisway.

As described above, in the present embodiment, the drive part of thepixel circuit shares the conversion use thin film transistor TFT1 in atime division manner together with the conversion part. Namely, thedrive part separates the conversion use thin film transistor TFT1 fromthe receiving part after completing the conversion of the signal currentIw and uses the same for drive and passes the drive current to the lightemitting element OLED through the channel in the state where the heldvoltage level is applied to the gate of the conversion use thin filmtransistor TFT1. Further, the drive part has a controlling means forcutting off the unnecessary current flowing through the light emittingelement OLED via the conversion use thin film transistor TFT1 at timesother than the drive. In the case of the present example, thecontrolling means controls the voltage between terminals of the twoterminal type light emitting elements OLED having the rectificationfunction by the anode line A and cuts off the unnecessary current.

The driving method of this circuit is as follows. The drive waveform isshown in FIG. 10. First, the first scanning line SCAN-A and the secondscanning line SCAN-B are brought to the selected state at the time ofwriting. In the example of FIG. 10, the first scanning line SCAN-A isset at a low level, and the second scanning line SCAN-B is set at a highlevel. Here, the current source CS of the current value Iw is connectedto the data line DATA, but in order to prevent the Iw from flowing viathe light emitting element OLED, the anode line A of the light emittingelement OLED is set at low level (for example GND or negative potential)so that the light emitting element OLED becomes the off state. By this,the signal current Iw flows through the TFT1. At this time, the gate andthe drain of the TFT1 are electrically short-circuited by the TFT4,therefore equation (5) stands, and the TFT1 operates in the saturatedregion. Accordingly, the voltage Vgs given by equation (3) is createdbetween the gate and the source thereof. Next, the first scanning lineSCAN-A and the second scanning line SCAN-B are brought to the unselectedstate. In more detail, first, the second scanning line SCAN-B is broughtto the low level and the TFT4 is brought to the off state. By this, theVgs created in the TFT1 is held at the capacity C. Next, by setting theSCAN-A at the high level and bringing the TFT3 to the off state, thepixel circuit and the data line DATA are electrically cut off, andtherefore the writing to another pixel can be carried out via the dataline DATA after that. Here, the data supplied by the current source CSas the signal current Iw must be valid at a point of time when thesecond scanning line SCAN-B becomes unselected, but may be set at anyvalue (for example write data of the next pixel) after that. Then, theanode line A is brought to the high level. The Vgs of the TFT1 is heldby the capacitor C, therefore if the TFT1 operates in the saturatedregion, the current flowing through the TFT1 coincides with Iw inequation (3). This becomes the drive current Idrv flowing through thelight emitting element OLED. That is, the signal current Iw coincideswith the drive current Idrv of the light emitting element OLED. In orderto operate the TFT1 in the saturated region, a sufficient positivepotential may be given to the anode line A so that equation (5) stillstands even if the voltage drop at the light emitting element OLED isconsidered. According to the above drive, the current Idrv flowingthrough the light emitting element OLED correctly coincides with Iwwithout being affected by variations in the characteristics of the TFT.

FIG. 11 is a modification of the pixel circuit shown in FIG. 9. In FIG.11, there is no anode line as in FIG. 9. The anode of the light emittingelement OLED is connected to the constant positive potential Vdd, whilea P-channel transistor TFT5 is inserted between the drain of the TFT1and the cathode of the light emitting element OLED. The gate of the TFT5is controlled by the drive line drv arranged in units of the scanninglines. The object of insertion of TFT5 is prevention of the flow of thesignal current Iw via the light emitting element OLED by setting thedrive line drv at a high level and bringing the TFT5 to the off state atthe time of writing data. After the writing is ended, the drv is broughtto the low level, the TFT5 is brought to the on state, and the drivecurrent Idrv flows through the light emitting element OLED. The rest ofthe operation is similar to that of the circuit of FIG. 9.

The present example includes the TFT5 connected to the light emittingelement OLED in series and can cut off the current flowing to the lightemitting element OLED in accordance with the control signal given to theTFT5. The control signal is given to the gate of the TFT5 included ineach pixel on the identical scanning line via the drive line drvprovided in parallel to the scanning line SCAN. In the present example,the TFT5 is inserted between the light emitting element OLED and theTFT1, and the current flowing through the light emitting element OLEDcan be turned on or off by the control of the gate potential of theTFT5. According to the present example, the emission of light of eachpixel is achieved for the amount of time where the TFT5 is on by a lightemission control signal. When defining the on time as τ and the time ofone frame as T, the ratio in time when the pixel is emitting light, thatis, the duty, becomes approximately τ/T. A time average brightness ofthe light emitting element changes in proportional to this duty.Accordingly, by changing the on time τ by controlling the TFT5, it isalso possible to variably adjust the screen brightness of the EL displayconveniently and in a wide range.

As described above, in the present example, the controlling meanscomprises the control use thin film transistor TFT5 inserted between theconversion use thin film transistor TFT1 and the light emitting elementOLED. The control use thin film transistor TFT5 becomes nonconductiveand separates the conversion use thin film transistor TFT1 and the lightemitting element OLED when the light emitting element OLED is not drivenand switches to the conductive state at the time of drive. Further, thiscontrolling means can control the brightness of each pixel bycontrolling the ratio between the off time for which the drive currentis cut off and the light emitting element OLED is placed in thenon-light emitting state when the OLED is not to be driven and the ontime for which the drive current is passed and the light emittingelement OLED is placed in the light emitting state when the OLED is tobe driven. According to the present example, before the brightnessinformation of the next scanning line cycle (frame) is newly writtenafter writing the brightness information into the pixels in units of thescanning lines, the display device can extinguish the light emittingelements contained in the pixels in units of the scanning linestogether. This means that the time from the lighting to theextinguishing of the light emitting elements after the writing of thebrightness information can be adjusted. Namely, it means that the ratio(duty) of the light emitting time in one scanning line cycle can beadjusted. The adjustment of the light emitting time (duty) correspondsto the adjustment of the drive current supplied to each light emittingelement. Accordingly, it is possible to adjust the display brightnessconveniently and freely by adjusting the duty. A further important pointresides in that the drive current can be equivalently made large byadequately setting the duty. For example, when the duty is set at 1/10,even if the drive current is increased to 10 times, an equivalentbrightness is obtained. If the drive current is made 10 times large,also the signal current corresponding to this can be made 10 timeslarger, and therefore it is not necessary to handle a weak currentlevel.

FIG. 12 is another modification of the pixel circuit shown in FIG. 9. InFIG. 12, a TFT6 is inserted between the drain of the TFT1 and thecathode of the light emitting element OLED, a TFT7 is connected betweenthe gate and the drain of the TFT6, and the gate thereof is controlledby the second scanning line SCAN-B. An auxiliary capacity C2 isconnected between the source of the TFT7 and the GND potential. Thedriving method of this circuit is basically the same as the case of thepixel circuit of FIG. 9, but will be explained below. Note that, thedrive waveform is similar to that of the case of FIG. 10. First, at thetime of writing, when the first scanning line SCAN-A and the secondscanning line SCAN-B are brought to the selected state in the statewhere the anode line A arranged in units of the scanning lines isbrought to the low level (for example GND or negative potential) and thecurrent is prevented from flowing through the OLED, the signal currentIw flows through the TFT1 and TFT6. Since the gates and the sources areshort-circuited by the TFT4 and TFT7, the two TFTs operate in thesaturated region. Next, the first scanning line SCAN-A and secondscanning line SCAN-B are brought to the unselected state. By this, theVgs previously created in the TFT1 and the TFT6 are held by thecapacitor C and the auxiliary capacitor C2. Next, by bringing the firstscanning line SCAN-A to the off state, the pixel circuit and the dataline DATA are electrically cut off, therefore the writing to anotherpixel can be carried out via the data line DATA after that. Then, theanode line A is set at a high level. Since the Vgs of the TFT1 is heldby the capacitor C, if the TFT1 operates in the saturated region, thecurrent flowing through the TFT1 coincides with Iw of equation (3). Thisbecomes the current Idrv flowing through the light emitting elementOLED. That is, the signal current Iw coincides with the drive currentIdrv of the light emitting element OLED.

Here, an explanation will be made of the function of the TFT6. In thepixel circuit of FIG. 9, as mentioned before, both of the signal currentIw and the drive current of the light emitting element OLED aredetermined by the TFT1, therefore Iw=Idrv stood by equation (3) andequation (4). Note, this is true when assuming a case where the currentIds flowing through the TFT1 is given by equation (1) in the saturatedregion, that is, Ids does not depend on the voltage Vds between thedrain and the source. Nevertheless, in an actual transistor, even if Vgsis constant, the larger Vds, the larger Ids in a certain case. This isdue to the so-called short channel effect where a pinchoff point in thevicinity of the drain moves to the source by an increase of the Vds, andan effective channel length is reduced, or a so-called back gate effectwhere the potential of the drain exerts an influence upon the channelpotential, and the conduction rate of the channel changes, and so on. Inthis case, the current Ids flowing through the transistor becomes forexample as in the following equation.Ids=μ·Cox·W/L/2(Vgs−Vth)²*(1+λ·Vds)  (9)

Accordingly, Ids will depend on Vds. Here, λ is a positive constant. Inthis case, in the circuit of FIG. 9, Iw does not coincide with Idrvunless Vds is not identical between the time of the writing and the timeof the drive.

As opposed to this, the operation of the circuit of FIG. 12 will beconsidered. When paying attention to the operation of the TFT6 of FIG.12, the drain potential thereof is not generally identical between thetime of the writing and the time of the drive. For example, where thedrain potential at the time of the drive is higher, the Vds of the TFT6becomes larger. When inserting this in equation (9), even if Vgs isconstant between the time of the writing and the time of the drive, Idsis increased at the time of the drive. In other words, Idrv becomesbigger than Iw, and the two do not coincide. However, the Idrv flowsthrough the TFT1, therefore, in that case, the voltage drop at the TFT1becomes large and the drain potential thereof (source potential of theTFT6) rises. As a result, Vgs of the TFT6 becomes small. This acts in adirection reducing the Idrv. As a result, the drain potential of theTFT1 (source potential of the TFT6) cannot largely fluctuate. Whenpaying attention to the TFT1, it is seen that Ids does not largelychange between the time of the writing and the time of the drive.Namely, Iw and Idrv will coincide with a remarkably high precision. Inorder to perform this operation better, it is good if the dependency ofIds with respect to Vds is made small in both of the TFT1 and TFT6,therefore desirably both transistors are operated in the saturatedregion. At the time of writing, the gate and the drain areshort-circuited in both of the TFT1 and TFT6. Therefore, regardless ofthe brightness data written, the two operate in the saturated region. Inorder to operate them also at the drive, a sufficient positive potentialmay be given to the anode line A so that the TFT6 still operates in thesaturated region even if the voltage drop at the light emitting elementOLED is considered. By this drive, the current Idrv flowing through thelight emitting element OLED more correctly coincides with the Iw thanthe embodiment of FIG. 9 without being affected by variations in thecharacteristics of the TFT. As described above, the drive part of thepresent example has TFT6, TFT7, and C2 as potential fixing means forfixing the potential of the drain with reference to the source of theconversion use thin film transistor TFT1 for stabilizing the currentlevel of the drive current flowing to the light emitting element OLEDthrough the conversion use thin film transistor TFT1.

FIG. 13 is another embodiment of the pixel circuit according to thepresent invention. The characterizing feature of this pixel circuitresides in that, in the same way as FIG. 9, FIG. 11, and FIG. 12, thetransistor TFT1 per se with the signal current Iw flowing therethroughcontrols the current Idrv flowing through the light emitting elementOLED, but in FIG. 13, the light emitting element OLED is connected tothe source side of the TFT1. Namely, the drive part of the present pixelcircuit includes the thin film transistor TFT1 provided with the gate,drain, and the source and passes the drive current passing between thedrain and the source to the light emitting element OLED in accordancewith the level of the voltage applied to the gate. The light emittingelement OLED is a two-terminal type having an anode and a cathode, andthe anode is connected to the source. On the other hand, the drive partof the pixel circuit shown in FIG. 9 includes the thin film transistorprovided with the gate, drain, and the source and passes the drivecurrent passing between the drain and the source to the light emittingelement in accordance with the level of the voltage applied to the gate.The light emitting element is the two-terminal type having an anode anda cathode, and the cathode is connected to the drain.

The pixel circuit of the present example comprises, other than the TFT1,a transistor TFT3 for connecting or cutting off the pixel circuit andthe data line DATA by the control of the first scanning line SCAN-A, atransistor TFT4 for short-circuiting the gate and the drain of the TFT1during the writing period by the control of the second scanning lineSCAN-B, a capacitor C for holding the gate potential of the TFT1 evenafter the end of the writing, a P-channel transistor TFT5 insertedbetween the drain of the TFT1 and the power supply potential Vdd, andthe light emitting element OLED. In FIG. 13, one terminal of thecapacitor C is connected to the GND, and the Vgs of the TFT1 is held atschematically the same value between the time of the writing and thetime of the drive. Note that, the gate of the TFT5 is controlled by thedrive line drv. The object of the insertion of the TFT5 is to bring theTFT5 into the off state by setting the drive line drv at the high levelat the time of writing data and pass all of the signal current Iwthrough the TFT1. After the writing is ended, the drv is brought to thelow level, the TFT5 is brought to the on state, and the drive currentIdrv is passed through the light emitting element OLED. In this way, thedriving method is similar to that of the circuit of FIG. 11.

FIG. 14 is a modification of the pixel circuit shown in FIG. 13. In FIG.13 and FIG. 14, the difference resides in that one terminal of thecapacitor C is connected to the GND in FIG. 13, but is connected to thesource of the TFT1 in FIG. 14, but in both cases, there is no functionaldifference in the point that the Vgs of the TFT1 is held atschematically the same value between the time of the writing and thetime of the drive.

FIG. 15 is a more developed example of the pixel circuit shown in FIG.5. The present pixel circuit includes an adjusting means for downwardlyadjusting the voltage level held by the converting part and supplyingthe same to the drive part to tighten the black level of the brightnessof each pixel. Concretely, the drive part includes a thin filmtransistor TFT2 having a gate, drain, and source and an adjusting meansprovided with a constant voltage source E for raising the bottom of thevoltage between the gate and the source of the thin film transistor TFT2and downwardly adjusting the level of the voltage applied to the gate.Namely, it tightens the black level by connecting the source of the TFT2to the potential E slightly higher than the source potential of theTFT1.

FIG. 16 is a modification of the pixel circuit shown in FIG. 15. In thepresent example, the adjusting procedure is comprised by an additionalcapacitor C2 connected to the gate of the thin film transistor TFT2 andthe second scanning line SCAN-B and downwardly adjusts the voltage levelto be held at the capacitor C for applying the same to the gate of thethin film transistor TFT2. Namely, when switching the second scanningline SCAN-B to the low level and bringing it to the unselected state,the gate potential of the TFT2 can be slightly lowered by the functionof the capacitor C2. As described above, in the present display device,the scanning line SCAN-A for selecting the pixel and the data line DATAgiving the brightness information for driving the pixel are arranged inthe matrix state. Each pixel includes the light emitting element OLEDhaving the brightness changing according to the amount of the suppliedcurrent, the writing means (TFT1, TFT3, C) controlled by the scanningline SCAN-A and writing the brightness information given from the dataline DATA to the pixel, and the driving means (TFT2) for controlling theamount of the current supplied to the light emitting element OLED inaccordance with the written brightness information. The brightnessinformation is written into each pixel by applying the electric signalIw in accordance with the brightness information to the data line DATAin the state where the scanning line SCAN A is selected. The brightnessinformation written in each pixel is held at each pixel even after thescanning line SCAN-A becomes unselected. The light emitting element OLEDof each pixel includes the adjusting means (C2) capable of maintainingthe lighting with the brightness in accordance with the held brightnessinformation, downwardly adjusting the brightness information written bythe writing means (TFT1, TFT3, C), and supplying the same to the drivemeans (TFT2) and can tighten the black level of the brightness of eachpixel.

FIG. 17 is a modification of the pixel circuit shown in FIG. 15. In thepresent example, the adjusting procedure downwardly adjusts the level ofthe voltage to be applied to the gate of the TFT2 by adjusting thepotential of one end of the capacitor C when holding the voltage levelconverted by the TFT1 at the capacitor C. Namely, by controlling thesource potential control line S connected to one end of the capacitor C,the black level is tightened. This is because the gate potential of theTFT2 is slightly lowered by the function of the capacitor C when settingthe potential control line S at a lower potential than that at thewriting. The potential control line S is provided in units of thescanning lines and controlled. The potential control line S is broughtto an “H” level during the writing and brought to an “L” level after theend of the writing. When defining an amplitude as ΔVs and defining thecapacity existing at the gate of the TFT2 (gate capacity, otherparasitic capacity) as Cp, the gate potential of the TFT2 is lowered byexactly ΔVg=ΔVs*C/(C+Cp), and Vgs becomes small. The absolute values ofthe H and L potentials can be freely set.

FIG. 18 is another embodiment of the pixel circuit according to thepresent invention. In the circuit of the present example, the fetch usethin film transistor TFT3 and the switch use thin film transistor TFT4are configured as the identical conductivity type (PMOS in FIG. 18).Then, in the present example, as shown in FIG. 18, it is also possibleto connect their gates to the common scanning line SCAN in the writeoperation and control them by the common signal. In the device displayin this case, the scanning line drive circuit B23 in the display deviceshown in FIG. 7 is unnecessary.

FIG. 19 is a modification of the pixel circuit shown in FIG. 18. In thepresent example, in the same way as the circuits shown in FIG. 5, FIG.8, FIG. 9, and FIG. 11 to FIG. 17, the gates of the fetch use thin filmtransistor TFT3 and switch use thin film transistor TFT4 configured bythe same conductivity type P-channel TFT are connected to differentscanning lines, that is, the first scanning line SCAN-A and the secondscanning line SCAN-B, and separately controlled. The reason why they areseparately controlled in this way is that, if the TFT3 and the TFT4 arecontrolled by the common signal as in the example of FIG. 18, thefollowing inconvenience sometimes occurs.

When the write operation with respect to the pixel on a certain scanningline is terminated, at the rise of the level of the scanning line SCANin the example of FIG. 18, the impedance of the TFT3 is inevitablyincrease and finally actually becomes infinitely large, that is, the offstate. Accordingly, in this step, the potential of the data line DATAgradually rises, but at a point of time when it rises to a certaindegree, the current source for driving the data line DATA loses theconstant current property, and the current value is decreased.

As a concrete example, an example where the data line DATA is driven bya PNP transistor BIP1 as in FIG. 18 is considered. When the currentflowing through the base is the constant value Ib and a currentamplification rate of a transistor IBIP1 is β, if a certain degree ofthe voltage (for example 1V) is applied between the collector and theemitter of the transistor BIP1, the transistor BIP1 operates assubstantially a constant current source, and a current of a magnitude ofIw=βIb is supplied to the data line DATA. However, at the end of thewrite operation, when the impedance of the TFT3 rises, the potential ofthe data line rises, and when the transistor BIP1 enters into thesaturated region, it loses the constant current property, and the drivecurrent is decreased from βIb. At this time, if the TFT4 is in the onstate, this decreased value of the current flows through the TFT1, andthe intended value of the current will not be correctly written.

Accordingly, more desirably the TFT3 and the TFT4 are controlled by thedifferent signal lines, that is, the first scanning line SCAN-A and thesecond scanning line SCAN-B, and the TFT4 is brought to the off statepreceding the TFT3 at the end of the write operation. In the pixelcircuit according to the present invention, the TFT3 and the TFT4 do nothave to be the same conductivity type as in the examples mentionedbefore. The pixel circuit may be configured so that the TFT3 and theTFT4 are the identical or different conductivity types, their gates arecontrolled by the different scanning lines such as the SCAN-A and theSCAN-B, and the TFT4 is brought to the off state preceding the TFT3 atthe end of the write operation. This is true also for the examplesexplained before by referring to the drawings.

Further, when the TFT3 and the TFT4 are controlled by the differentscanning lines SCAN-A and SCAN-B, after the end of the write operation,the TFT4 is brought to the on state by the operation of the secondscanning line SCAN-B, and the pixels can be extinguished in units of thescanning lines. This is because the gate and the drain of the TFT1 andthe gate of the TFT2 are connected, so the gate voltage of the TFT2becomes the threshold value of the TFT1 (this is almost equal to thethreshold value of the TFT2), and both of the TFT1 and TFT2 become theoff state. In the waveform of the second SCAN-B, as shown in FIG. 20(B),it is also possible to give a pulse-like extinguishing signal, or it isalso possible to give a continuous extinguishing signal as SCAN-B′ shownin FIG. 20(C).

In this way, by changing the timing of the extinguishing signal, it ispossible to conveniently and freely change the brightness of the displaydevice. If the second scanning line SCAN-B is divided for each of thecolors of R, G, and B and they are separately controlled, the colorbalance can also be easily adjusted.

Further, when it is desired to obtain the same time average brightness,by reducing the ratio of the light emission period (duty), the drivecurrent of the light emitting element OLED can be made large. This meansthat a write current larger by that amount is handled. Therefore, therealization of the write drive circuit to the data line DATA becomeseasy, and also a write required time can be shortened. Further, byreducing the light emission duty to about 50% or less, the movingpicture image quality is improved.

Further, in the same way as the circuits shown in FIG. 5, FIG. 8, FIG.9, and FIG. 11 to FIG. 18, in the circuit of FIG. 19, the fetch use thinfilm transistor TFT3 and the conversion use thin film transistor TFT1are configured as different conductivity types. For example, where theconversion use thin film transistor TFT1 is the N-channel type, thefetch use thin film transistor TFT3 is configured as the P-channel type.This is for the following reason.

Namely, desirably the fluctuation of the potential of the data line isas small as possible when configuring the constant current drive circuitfor driving the data line. This is because, as mentioned before, if theamount of fluctuation of the data line potential is wide, the constantcurrent property is easily lost in the data line drive circuit. Inaddition, the amplitude of the scanning line SCAN-A for reliably turningon or off the TFT3 becomes large. This is disadvantageous in the pointof the consumed power.

Accordingly, desirably the voltage drop of the route reaching the groundpotential from the data line via the TFT3 and the TFT1 is small.Therefore, in contrast to the example of FIG. 19 wherein the TFT1 is anNMOS, the TFT3 is configured by a PMOS, and the voltage drop at the TFT3is suppressed small. Namely, the voltage drop at the TFT3 becomes themaximum when the value of the write current Iw is the maximum.Therefore, in order to suppress the amplitude of the data line small,the voltage drop at the TFT3 when the write current Iw is the maximumshould be made small. In the example of FIG. 19, when the write currentIw is large, the potential of the data line rises in accordance withthat, but the absolute value of the voltage between the gate and thesource of the TFT3 is increased along with that and the impedance of theTFT3 is lowered. Contrary to this, if the TFT3 is an NMOS, the largerthe write current Iw, the smaller the voltage between the gate and thesource, the greater the impedance of the TFT3, and the more easily arise of the data line potential is induced. Similarly, when the TFT1 isconfigured by a PMOS, the TFT3 is preferably configured by a NMOS.

Note that a practical configuration can be realized whether theconductivity type of the TFT4 is the same as or different from the TFT3,but if the TFT4 is given the same conductivity type as that of the TFT3,the first scanning line SCAN-A and the second scanning line SCAN-B areeasily driven by the common potential, so this is more desirable.

FIG. 21 is a modification of the pixel circuit shown in FIG. 19. Thepixel circuit according to the present example is similar to the pixelcircuit shown in FIG. 19 in terms of the equivalent circuit, but it isdifferent from the circuit of FIG. 19 in the point that the ratio W/Lbetween the channel width (W) and the channel length (L) of theconversion use thin film transistor TFT1 is set larger than the W/L ofthe drive use thin film transistor TFT2. The reason for setting the W/Lof the TFT1 larger than the W/L of the TFT2 in this way is for reliablyending the write operation. An explanation will be made of this below bygiving specific figures.

As practical numbers, when the maximum brightness is 200 cd/m², the sizeof the light emitting surface per pixel is 100 μm×100 μm=1e−8 m², andthe light emission efficiency is 2 cd/A, the drive current of the lightemitting element OLED at the maximum brightness becomes 200×1e−8/2=1 μA.When it is intended to control 64 tones, the current value correspondingto the minimum tone becomes about 1 μA/64=16 nA. It is extremelydifficult to correctly supply such a small current value. Further, theTFT1 operates in the state of high impedance, therefore a long time istaken for stabilization of the state of the circuit due to an influenceof a parasitic capacitance of the data line DATA, etc. The writeoperation sometimes cannot be terminated within the predeterminedscanning line cycle.

As shown in FIG. 21, if W/L of TFT1=100/10 and W/L of TFT2=5/20, theratio of W/L becomes 40, the write current to be supplied to the dataline DATA for obtaining the OLED drive current of 16 nA becomes 16nA×40=640 nA, which is a practical number, so the write operation can bereliably terminated. When the TFT1 and the TFT2 comprise a plurality oftransistors, the above calculation naturally should be carried out byconsidering an effective W/L.

FIG. 22 is a more developed example of the circuit shown in FIG. 19. Inthe present pixel circuit, a leak element LEK1 is connected between eachdata line DATA and the predetermined potential to try to speed up ofblack writing.

In the current write type pixel circuit, a case of writing “black”corresponds to a case where the write current is zero. At this time,when assuming that a “white” level, that is a relatively large current,is written into the data line in the scanning line cycle immediatelybefore that and as a result the data line potential has become arelatively high level, a long time is necessary for writing “black”immediately after that. The writing of “black” means that initialcharges stored in a capacitor Cd etc. of the data line are discharged,but when the data line potential is lowered and becomes in the vicinityof the threshold value of the TFT1, the impedance of the TFT1 becomeshigh, and as indicated by a characteristic curve <1> in FIG. 23 showingthe characteristic of the current flowing through the TFT1,theoretically the “black” writing is permanently not terminated. Inactuality, the write operation is carried out in a finite time,therefore this appears as a so-called “black float” phenomenon where the“black” level is not completely achieved. This lowers the contrast ofthe image.

Therefore, in the circuit of FIG. 22, the leak element LEK1, concretelythe NMOS transistor, is connected between the data line DATA and theground potential GND, and a constant bias is given as Vg. By this, asindicated by a characteristic curve <2> in FIG. 22, the “black” writingis reliably terminated. As the leak element LEK1, also a simple resistormay be used, but in that case, when the data line potential rises at the“white” writing, the current flowing through the resistor is increasedin proportion to that, and this induces the lowering of the currentflowing through the TFT1 and the degradation of the power consumption.Contrary to this, if an NMOS is operated in the saturated region, aconstant current operation is achieved, therefore such a bad influencecan be suppressed small. Note that, it is also possible to comprise theleak element by an TFT or comprise the same by an external partseparately from the TFT process.

FIG. 24 is a more developed example of the circuit shown in FIG. 19. Inthe present pixel circuit, an initial value setting element PRC1 isconnected between each data line DATA and the predetermined potential,and the initial value of the data line is set preceding the writeoperation by the operation of that element to speed up the writeoperation.

In the current write type pixel, there is a case where a long time isrequired when writing gray near black. In FIG. 25, a case where thepotential of the data line at the start of the write operation is 0V isshown. This can occur in the case where “black” is written in thescanning line cycle immediately before that, the case where thethreshold value Vth1 of the TFT1 of the written pixel is low, i.e. about0V, or similarly the case of the black writing, and the case where theleak element for the countermeasure of black float is provided.

In the conventional circuit, gray near “black”, that is, a very smallcurrent value, is written from 0V as the initial value, therefore a longtime is taken for reaching the balance potential VBLA. For example, asindicated by the characteristic curve <1> in FIG. 25, it can be alsoconsidered that the threshold value of the TFT1 is not reached withinthe predetermined write time, but in this case, the TFT2 also becomesthe off state, the gray cannot be correctly written, and the displayimage exhibits a so-called black crushed state.

In the circuit of FIG. 24, a PMOS transistor is connected between thedata line and the power supply potential Vdd as the initial valuesetting (precharging) element PRC1, and the first pulse is given at thefirst writing cycle as the gate potential Vg. By this pulse application,as indicated by the characteristic curve <2> in FIG. 25, the data linepotential rises to the threshold value Vth1 of the TFT1 or more andconverges thereafter at relatively a high speed toward the balancepotential VBLA determined by a balance between the write current Iw andthe operation of the TFT inside the pixel, so correct writing of thebrightness data at a high speed becomes possible. Note that, it is alsopossible to configure the precharge use element by a TFT or configurethe same by an external part separately from the TFT process.

FIG. 26 is another embodiment of the pixel circuit according to thepresent invention. In this circuit, unlike the circuits of the examplesmentioned before, the conductivity types of the TFT1 and the TFT2 areachieved by the P-channel type (PMOS). Along with this, for the abovereason, the TFT3 is configured as the N-channel type (NMOS) as aconductivity type different from that of the TFT1. The TFT4 isconfigured as the N-channel type (NMOS) as the identical conductivitytype to that of the TFT3 in consideration with the controllability.

In the circuit of FIG. 26, the two transistors TFT1 and TFT2 operate byequal gate-source voltages at the time of driving the light emittingelement OLED, but the drain-source voltages are not always equal. Inorder to achieve a correct proportion between the write current Iw andthe drive current of the light emitting element OLED, desirably the TFT2is operated in the saturated region as previously mentioned. On theother hand, in the case of an NMOS, generally an LDD (lightly dopeddrain) structure is employed in order to improve the withstand voltage.This is because, in this case, the drain current is easily influenced bythe drain-source voltage in the saturated region. In other words, theconstant current property tends to be inferior to an PMOS due to aserial resistance component by the LDD.

Accordingly, preferably the conversion use thin film transistor TFT1 andthe drive use thin film transistor TFT2 are configured by PMOSs.

The operation of this circuit is basically similar to that of thecircuit of FIG. 5 etc. except for the point that the polarities of theelements become reverse.

FIG. 27 shows another embodiment of the pixel circuit according to thepresent invention. Unlike the circuits of the examples mentioned above,this circuit is configured so that, in place of connecting the switchuse thin film transistor TFT4 between the drain and the gate of theconversion use thin film transistor TFT1, the drain and the gate of theTFT1 are directly connected, and the TFT4 is connected between aconnection point of them and the connection point between the gate ofthe TFT2 and the capacitor.

Also in this circuit of FIG. 27, basically the operation the same way asthat in the circuit of FIG. 5 etc. is possible. Further, also in thiscircuit, the TFT3 and the TFT4 may be identical or differentconductivity types, the gates of them are controlled by differentscanning lines such as the first scanning line SCAN-A and the secondscanning line SCAN-B, and the TFT4 brought to the off state precedingthe TFT3 at the end of the write operation. Further, as explained inrelation to FIG. 21, in order to reliably terminate the write operationin the predetermined scanning line cycle, desirably the size (W/L) ofthe TFT1 is set larger than the size of the TFT2.

INDUSTRIAL APPLICABILITY

As described above, by the current drive circuit according to thepresent invention and the display device using the same, it is possibleto pass a drive current Idrv correctly proportional (or corresponding)to the signal current Iw from a data line through a current drive typelight emitting element (organic EL element or the like) without beingaffected by variations in the characteristics of the active element (TFTetc.) By arranging a large number of pixel circuits including suchcurrent drive circuits in a matrix, each pixel can be made to correctlyemit light with the intended brightness. Therefore it is possible toprovide a high quality active matrix type display device.

1-165. (canceled)
 166. A method of driving a light emitting element fordriving a current-driven type light emitting element arranged at anintersecting portion of a data line supplying a signal current of acurrent level in accordance with brightness information and a scanningline supplying a selection pulse and emitting light by the drivecurrent, comprising: a receiving routine for fetching the signal currentfrom said data line in response to a selection pulse from said scanningline, a converting routine for converting a current level of the fetchedsignal current to a voltage level and holding the same, and a driveroutine for passing a drive current having a current level in accordancewith the held voltage level through the light emitting element, theconverting routine includes a routine using a conversion use insulatinggate type field effect transistor provided with a gate, a source, adrain, and a channel and a capacitor, one end of the capacitor beingconnected to the gate and another end of the capacitor being connectedto a potential control line, the potential control line being providedin units of the scanning lines, in the routine, the conversion useinsulating gate type field effect transistor creates the voltage levelconverted by passing the fetched signal current though the channel inthe receiving routine at the gate, and the capacitor holds voltage levelcreated at the gate, and the drive routine part shares the conversionuse insulating gate type field effect transistor together with theconverting part in a time division manner, and the drive routineseparates the conversion use insulating gate type field effecttransistor from the receiving part and uses the same for driving afterthe conversion of the signal current is completed and passes the drivecurrent to the light emitting element through the channel in a statewhere the held voltage level is applied to the gate of the conversionuse insulating gate type field effect transistor.